package leetcode._0134;

/**
 * 134. Gas Station
 * There are N gas stations along a circular route, where the amount of gas at station i is gas[i].
 *
 * You have a car with an unlimited gas tank and it costs cost[i] of gas to travel from station i to its next station (i+1). You begin the journey with an empty tank at one of the gas stations.
 *
 * Return the starting gas station's index if you can travel around the circuit once in the clockwise direction, otherwise return -1.
 *
 * Note:
 *
 * If there exists a solution, it is guaranteed to be unique.
 * Both input arrays are non-empty and have the same length.
 * Each element in the input arrays is a non-negative integer.
 * Example 1:
 *
 * Input:
 * gas  = [1,2,3,4,5]
 * cost = [3,4,5,1,2]
 *
 * Output: 3
 *
 * Explanation:
 * Start at station 3 (index 3) and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
 * Travel to station 4. Your tank = 4 - 1 + 5 = 8
 * Travel to station 0. Your tank = 8 - 2 + 1 = 7
 * Travel to station 1. Your tank = 7 - 3 + 2 = 6
 * Travel to station 2. Your tank = 6 - 4 + 3 = 5
 * Travel to station 3. The cost is 5. Your gas is just enough to travel back to station 3.
 * Therefore, return 3 as the starting index.
 * Example 2:
 *
 * Input:
 * gas  = [2,3,4]
 * cost = [3,4,3]
 *
 * Output: -1
 *
 * Explanation:
 * You can't start at station 0 or 1, as there is not enough gas to travel to the next station.
 * Let's start at station 2 and fill up with 4 unit of gas. Your tank = 0 + 4 = 4
 * Travel to station 0. Your tank = 4 - 3 + 2 = 3
 * Travel to station 1. Your tank = 3 - 3 + 3 = 3
 * You cannot travel back to station 2, as it requires 4 unit of gas but you only have 3.
 * Therefore, you can't travel around the circuit once no matter where you start.
 */

import java.util.HashMap;
import java.util.Map;

/**
 * https://leetcode.com/problems/gas-station/
 */
public class Solution {
    /**
     * 暴力解
     * @param gas
     * @param cost
     * @return
     */
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int length = gas.length;
        for(int i = 0; i < length; i++) {
            if(getLeavelGas(i, gas, cost) >= 0) {
                return i;
            }
        }
        return -1;

    }

    /**
     * 选择合适的起点
     * @param gas
     * @param cost
     * @return
     */
    public int canCompleteCircuit2(int[] gas, int[] cost) {
        int length = gas.length;
        HashMap<Integer, Integer> map = new HashMap<>();
        for(int i = 0; i < length; i++) {
           int temp = gas[i] - cost[i];
           if (temp >= 0) {
               map.put(i, temp);
           }
        }
        for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
            int index = entry.getKey();
            if (getLeavelGas(index, gas, cost) >= 0) {
                return index;
            }
        }
        return -1;

    }


    /**
     * 从当前站遍历到气量
     * @param startIndex
     * @param gas
     * @param cost
     * @return
     */
    public int getLeavelGas(int startIndex, int[] gas, int[] cost) {
        int length = gas.length;
        int initGas = 0;
        for(int i = 0; i < length; i ++) {
            int tempIndex = startIndex + i;
            if(tempIndex >= length) {
                tempIndex = tempIndex - length;
            }
            int total = initGas + gas[tempIndex] - cost[tempIndex];
            if (total < 0) {
                return -1;
            }
            initGas = total;
        }
        if (initGas < 0) {
            return -1;
        }
        return initGas;
    }
}
